The present invention relates to a tunable optical filter device comprising a wavelength selective optical filter comprising a grating structure arranged in a waveguide structure.
Important applications for wavelength tunable optical filters include optical transmission technology and optical switching within telecommunications. Further fields of application include optical connections, for example between printed circuit boards, partial systems etc. within a computer or a telephone station. Particularly, wavelength multiplexing can have significant importance within telenets, partly due to the difficulties which arise from dispersion in optical fibers in the construction of transmission systems intended to be used for long distances (for example more than 50 km) and with bit frequencies (per single wavelength channel) higher than 20-40 Gb/s. Furthermore, the flexibility becomes greater with wavelength multiplexing in the construction of optical exchanges, since different channels can be separated by the use of different wavelengths. With the help of filters as mentioned above, it is for example possible to filter out one out of many wavelength channels and the possibilities increase for both transmission and switching of a large information bandwidth and the filters also find application on both the receiver side and in the exchanges (switches) wherein different channels can be separated from each other.
It is thus desirable that the system in which the filters are used should be able to handle as many channels as possible, i.e. comprise an information bandwidth which is as large as possible.
In a number of different situations it is also advantageous to be able to fabricate tunable optical filters in the same material system as lasers, detectors, optical switch matrixes and modulators, particularly InGaAsP/InP for long wavelengths, 1300-1600 nm, and GaAs/AlGaAs for wavelengths between 800 and 900 nm.
It is a problem that, for physical reasons, the tuning range for such filters is limited. The exact value is therefore dependant on the choice of material composition (bandgap) in the waveguide material, the optical confinement factor (of the waveguide material thickness and the refractive index), doping level in the waveguide material and in the enclosing material and the thermal dissipation capacity of the component (thermal resistance). The greater the tuning range is, the more channels can be used. In a wavelength multiplexed system, the total information bandwidth (number of wavelength channels times bit velocity/channel) is proportional to the tuning range of the filter. Normally the tuning range of an optical filter is up to about 5 to 15 nm. It is therefore of significance to be able to obtain a tuning range particularly for a given combination of waveguide and material parameters which is as large as possible.
A number of solutions have been suggested for increasing the useable wavelength range of tunable filters, particularly waveguide based grating filters.
A number of different types of filters have been presented which are made in different material systems, such as glass, semi-conductors, polymers and litiumniobat.
One kind of filter comprises a single-mode waveguide with a grating which is etched into the waveguide, either in the waveguide layer itself or in a separate grating layer which can be separated from the waveguide layer by a distance of about some 100 nm. These filters can be used either as wavelength selective components per se or they can form part of the components of wavelength selective DBR-lasers (Distributed Bragg Reflector). A grating filter of this kind functions in such a way that only light within a narrow wavelength range centered around .lambda..sub.0 is reflected and goes backwards in the waveguide, whereas the rest passes in the forward direction. By placing the high index material (InGaAsP or GaAs) in a P-I-N-transition with P- and N- doped low index layers above and below (double heterobarrier structure) a current can be injected. Using this current, a change, .DELTA.n, in refractive index n of the waveguide material is caused. This change in refractive index implies that the wavelength which is reflected shifts towards a shorter wavelength, .DELTA..lambda..sub.0 =(.lambda..sub.0 /n.sub.eff).multidot..DELTA.n.sub.eff. The resonance wavelength .lambda. .sub.0 is given by the grating period according to .lambda..sub.0 =2.multidot.n.sub.eff .multidot..LAMBDA., where A is the grating period. The change in effective index .DELTA.n.sub.eff is then approximately related to the change in the index .DELTA.n of the waveguide material according to .DELTA.n.sub.eff .apprxeq..DELTA.n.multidot..GAMMA., where .GAMMA. is the confinement factor which determines how much of the power of the optical mode lies in the waveguide layer. The selectivity and the reflectance of the filter is essentially given by the length of the grating, L, and grating coupling-coefficient, .kappa.. If the product .kappa..multidot.L exceeds 2 (approximately), a high reflectance is obtained, and the line width of the filter is uniquely defined by the value of .kappa.. To obtain a good performance of the filter when it is used as a discrete component, i.e. when it is not monolithically integrated with other components on the same substrate, dielectric layers have to be deposited on both end surfaces to minimize reflectivity. Examples of filters are further described in EP-A-0 391 334.
One of the problems with these filters lies in the fact that the tuning range, .DELTA..lambda..sub.0 max, is limited essentially to between 5 and 15 nm, the exact value depending on a number of factors such as material composition, optical confinement factor, doping level etc. The inherent physical limitation depends on the maximal chargecarrier density which can be injected to the active layer. Normally, it is desired to have a tuning range which is as large as possible, particularly greater than 5 to 15 nm since then among others more channels can be used in the system. In telecommunication systems a tuning range of about 20 to 30 nm is often desired which particularly corresponds to the wavelength window in which optical amplifiers, particularly so called erbium-doped fiber amplifiers or semiconductor laser amplifiers easily can be used.
In EP-A-0 391 334 a semiconductor laser-element is described which is tunable, wherein the so called Stark-effect is used. With this application, a decrease of the losses is achieved whereas no considerable increase of the tuning range is obtained.
EP-A-0 397 045 describes a semiconductor laser which can emit light in a larger wavelength range. A line-grating is used which at least comprises two different sub-gratings. These sub-gratings are, however, superposed on each other by use of double exposing on the same waveguide portion. The invention disclosed in said document relates to a laser and not to a separate optical filter. Furthermore, this device does not provide any satisfactory increase of the tuning range.